A new technology inspired by the self-healing powers of plants and animals may allow damaged planes to fix themselves on the fly and point out even minuscule holes to mechanics upon landing. If the technique pans out, then aircraft, wind turbines and perhaps even spaceships of the future may boast embedded circulatory systems with an epoxy resin that can bleed into holes or cracks and then fluoresce under ultraviolet light to mark the damage like a bruise during follow-up inspections.

The system could be a particular boon for lightweight, plastic-based composites known as fiber-reinforced polymers. Such polymers have recently grown in popularity with aircraft, spacecraft, automotive and wind-turbine manufacturers, who use the materials like protective layers of skin.

“Their Achilles heel is that they are quite susceptible to damage that is often undetectable to the eye,” said Ian Bond, an aerospace engineer at Bristol University in the United Kingdom. “Users of composites spend a lot of time trying to detect this damage and worrying about what happens when it grows.”

With funding from the Engineering and Physical Sciences Research Council, Bond and his collaborators have set out to compensate for the flaw with hollow glass fibers inspired by biological systems. “Most natural materials have an ability to heal and look after themselves when they’re damaged,” he said. With a similarly arranged network of vessels at vulnerable spots like the underbelly, doors, hatchways, wheel wells and wing bottoms, he reasoned, so might an aircraft.

Like clotting blood
At its base, the hierarchical system his team designed boasts a two-part epoxy system. The epoxy and a hardener fill adjacent hollow glass fibers that, when broken due to a debris strike or other damage, release their contents and mix to form a plug, somewhat akin to clotting blood. Matched pairs of those filled glass fibers are arranged within the plane’s structural skin, a larger network of carbon fibers embedded in stacked layers of plastic.

Adding a fluorescent dye to the epoxy could mark patched-up damage in need of more permanent repairs, though it would work only on fairly translucent surfaces. “The analogy is very much a bruise,” Bond said. “You hurt yourself and you know you’ve been hurt.”

For black carbon fiber composites not amenable to a color change, magnetic nanoparticles could be added to the epoxy and a handheld magnet-based scanner used to detect changes in their relative distribution due to a newly formed epoxy plug.

Highlighting both the difficulty and desire to effectively detect aircraft damage, other scientists have turned to an infrared camera that points out defects below the surface, ultrasonic waves that monitor the growth of defects that could become cracks, and optical fiber sensors that measure strains and temperatures within the composite structure in real time.

Early tests promisingAlthough Bond’s team has yet to test its self-healing system on aircraft, the epoxy network has performed well in standard “drop-weight tests” designed to simulate the effect of a dropped tool or kicked up runway debris. After the impact, tests suggest the bleeding epoxy can restore between 80 percent to 90 percent of the damaged surface’s original compression strength, which measures the ability of a vertically positioned panel to withstand being squeezed.

Bond said the repair also reduces the risk that the aircraft’s composite layers will detach from each other, a structural issue that can lead to what’s known as low-failure mode.

“If you think of a pack of playing cards, if they’re all stuck together then it’s like a rigid block,” he said. “If they’re not stuck together, then they all behave independently.” The epoxy plug restores some of the block-like rigidity and helps prevent the individual movement of separate layers, he said.

Bond cautioned that any commercial applications would require rigorous validation and substantial industrial backing. Nonetheless, he said the self-healing system’s safety benefits could promote more widespread acceptance of fiber-reinforced polymer composites in aircraft, effectively reducing the weight of planes and thus both their fuel needs and carbon emissions. Both Airbus and Boeing have expressed preliminary interest, he said, as has the United Kingdom’s Ministry of Defence.

“Certainly within 5 to 10 years, we’d like to see these sorts of materials on structures,” Bond said.

The renewable energy industry also might benefit from such self-healing surfaces. Wind turbines, Bond noted, are essentially wings, and European officials have clamored to locate more of them in offshore locations, increasing the expense of sending out crews for maintenance and repair. A turbine that could heal itself after wear and tear or impacts with, say, small birds, could dramatically lower costs.

Bond’s team also has explored how the system might be deployed in space, and he said the payback could be significant for high-risk expeditions, though he conceded extensive safety testing would be required and the technique’s finite nature would have to be addressed: “Once you use up the healing function, it’s gone.”

Current limitations
Two engineering experts said Bond’s idea struck them as innovative, but both expressed doubts about its practicality.

Steven Schneider, a professor of aeronautics and astronautics engineering at Purdue University, said he believed the system might work, though he questioned the useful shelf life of the capsule-enclosed epoxy and hardener and how much undesirable weight they would add to an aircraft.

Paul Fischbeck, a risk analyst and professor of engineering and public policy at Carnegie Mellon University in Pittsburgh, called the technology a “neat idea,” but questioned whether it would result in a significant risk reduction. He also expressed concern over whether the benefits would outweigh the extra cost and potential structural impacts of adding glass capsules to the existing composite fibers.

Even restoring the fiber material to 80 percent to 90 percent of its original strength may not be enough to pass muster, he said, potentially requiring engineers to increase the safety margins of the original design to offset the 10 percent to 20 percent loss.

For some military applications, “where the risks are much greater and getting back to friendly territory is critical,” Fischbeck said the self-healing technology might provide a better cost-benefit ratio.

Bond conceded that his team’s design needs to minimize the structural impact of adding the epoxy system, such as excessive weight, even as it seeks to combine the best features of nature’s original sources of inspiration. “Plants generally can take a lot of damage. You can take an axe to a tree and it will live,” he said. “You can’t do the same with many animals.”

If plants possess a more redundant vasculature that makes it more robust, however, most animals are more efficient. “You want robustness, but you also want efficiency,” Bond said.

A next-generation system now in development in Bond’s lab might provide some of both while overcoming current limitations. How? By incorporating a circulatory system that allows the epoxy and hardener to flow through the network and be replenished for repeated healing.

Reminiscent of blood pumped through veins and arteries, the system has been embedded in a foam sandwich set between two plastic-and-carbon layers. Initial impact tests suggest the network’s self-healing capabilities are working well, though as yet, it still lacks anything that could be likened to perhaps the best-known distributor of all: a beating heart.